Session J17: Frank J. Padden Jr. Award Symposium

Is chain end retardation responsible for anomalous dependence of crystal growth rate on molecular weight?

We present results on a robust anomaly in the crystallisation of blends of poly(ethylene oxide) (PEO) of differing molecular weight, M$_w$. Previous studies probing PEO blends have mainly focused on blends of low M$_w$, for which integral chain folding is important, with very high M$_w$. The PEO samples used in this study consist of a blend with both M$_w$'s well above the integral chain folding limit. In general, one would expect that such blends should show a monotonic decrease in spherulite growth rate, G, as the blend contains more high-M$_w$ component. Our results however show a clear non-monotonic G, with a minimum in a plot of G as a function of the volume fraction. Surprisingly, blending a small amount of lower- M$_w$ PEO into a higher- M$_w$ PEO slows the growth kinetics. We present a model that attributes this behavior to a chain end retardation mechanism.   

Reptation dynamics of single-walled carbon nanotubes in a permanent network

Single-walled carbon nanotubes (SWCNTs) are an ideal system of semiflexible filaments with tunable bending stiffness. By exploiting their near-infrared fluorescence, we image directly the motion of SWCNTs in a network (agarose gel). We determine the SWCNT diameter (and bending stiffness) spectroscopically, and we control the network pore size by changing the agarose concentration. Image analysis shows clearly that SWCNTs move by reptation through the pore network. We quantify the dependence of SWCNTs mobility on SWCNT bending stiffness, length and pore sizes. Our results show conclusively that, even when the SWCNT length is much smaller than the persistence length, the flexibility of filaments enhances rotational diffusion. These results confirm earlier predictions of Odijk (1983), and show that the Doi-Edwards scaling fails to capture the filaments' motion. This study provides a fundamental understanding of reptation dynamics of semiflexible filaments.   

Effects of Gradient Sequencing in Copolymers for Unusual, Tunable Properties and Phase Behavior

The sequencing of comonomers in gradient copolymers provides a novel, additional degree of freedom for controlling copolymer properties and phase behavior. With gradient copolymers of moderate segregation strength, we demonstrate that it is possible to substantially tune the glass transition temperature breadth through gradient sequence design. Transition breadths from 54 to 101 \r{ }C were achieved and these changes were correlated directly to predictions of composition variation from self-consistent mean-field theory. We also explore the influence of comonomer sequence on phase behavior using melt rheology, verifying the high level of temperature sensitivity predicted by theory; a gradual change from solid-like to liquid-like behavior was observed over a temperature range of 80 \r{ }C. Finally, experiments also expose a rare combination of upper and lower critical ordering transitions in gradient copolymers which are unattainable in block copolymers of the same system, demonstrating the potential for more complex phase diagrams through gradient sequencing.   

Development of universal coatings based on functional silicones

The development of a universal polymer coating prepared by the chemical coupling of trichlorosilane (TCS) to the vinyl groups of poly(vinylmethyl siloxane) (PVMS) is reported. The resultant PVMS-TCS material can be deposited as a functional organic layer on a variety of substrates ranging from hydrophobic to hydrophilic. The hardness of the material can likewise be regulated by adjusting the TCS concentration. Several case studies demonstrating the remarkable properties of these PVMS-TCS functional coatings will be presented. In particular, PVMS-TCS is found to serve as a convenient precursor for the deposition of organosilanes and the subsequent growth of polymer brushes, even on hydrophobic surfaces. In addition, PVMS-TCS can also be employed as a compatibilizing medium for functional silicone elastomer coatings composed of multilayered structures. The physical and chemical characteristics of these versatile PVMS-TCS coatings have been interrogated by a battery of experimental probes, including optical microscopy, near-edge x-ray absorption fine structure spectroscopy, x-ray photoelectron spectroscopy, Fourier-transform infrared spectroscopy, contact-angle measurements, atomic force microscopy, ellipsometry, and surface nanoindentation.   

Probing the orientation dynamics of multi-walled carbon nanotube composites via x-ray scattering methods

We have used in situ x-ray scattering methods to probe changes in orientation of multi-walled carbon nanotube (MWNT) composites in shear flow. These studies have been done using model nanocomposites prepared by dispersing MWNTs in Newtonian polymer matrix, to isolate effects of MWNT behavior from polymer elasticity. Structural measurements were made in a range of concentration in the flow-gradient (1-2) and flow-vorticity (1-3) planes of shear flow. The measurements revealed shear rate dependent MWNT orientation behavior; increased structural anisotropy with increasing shear rate. The degree of anisotropy was observed to decrease with particle concentration and aspect ratio. The induced anisotropy changed from being dominated by individual MWNT orientation in dilute suspensions to being dominated by deformations of MWNT networks in concentrated suspensions. A large, rapid loss of induced anisotropy was seen when flow was turned off due to dissipation of elastic energy stored in ``sticky'' MWNT contacts. These observations suggest that MWNT bending is central to the dynamic behavior of these suspensions under flow.   

Visible and NIR Polyelectrochromism in Polymer Acid Doped Polyaniline

Switching between the intermediate and fully reduced states of polyaniline (PANI) results in chromic transitions between green and transparent, respectively, while complete oxidation results in an additional transition to violet. Until now, efforts to exploit PANI's polyelectrochromicity have been hindered by conflicting requirements for stable switching. The green-transparent transition requires acidic conditions due to the proton dependence of PANI's intermediate form. PANI, however, undergoes hydrolysis when completely oxidized at these conditions; its violet form has therefore been elusive. By template polymerizing aniline on poly(2-acrylamido-2-methyl-1-propanesulfonic acid), PAAMPSA, we demonstrate sustained PANI electroactivity beyond pH 9. No longer limited by the requirement for acidic conditions, we show, for the first time, stable and reversible switching between the three oxidation states of PANI in aqueous media. Further, PANI-PAAMPSA exhibits large NIR contrast between its intermediate and fully reduced states, opening the door for PANI-based electrochromic devices operating in the NIR as well as visible range.   

The confining potential when a biopolymer filament reptates

Using single-molecule fluorescence imaging, we track Brownian motion perpendicular to the contour of tightly-entangled F-actin filaments and extract the confining potential. The chain localization presents a small-displacement Hookean regime followed by a large regime where the implied restoring force is independent of displacement. The implied heterogeneity characterized by a distribution of tube diameter is discussed and quantified. The concept of irregular, polydisperse entanglement length enables quantitative prediction of macroscopic properties such as the plateau modulus and by rational extension could benefit other problems characterized by Brownian motion through meshes. Work performed in collaboration with Juan Guan, Stephen Anthony, Kejia Chen, Sung Chul Bae and Ken Schweizer.   

Multi-Scale Morphologies of Poly(ethylene glycol) - Based Sulfonated Ionomers

A series of Li, Na, and Cs-neutralized sulfonated polyester ionomers with well-defined poly(ethylene glycol) (PEG) spacer lengths have been investigated by \textit{in situ} X-ray scattering from 25 $^{\circ}$C to 150 $^{\circ}$C. At room temperature, the ``ionomer peak'' (q=1-5 nm$^{-1})$ is absent in Na and Cs-neutralized ionomers, while Li-neutralized ionomers shows a peak at q = 2-3 nm$^{-1}$, reminiscent of conventional ionomers. As the length of the PEG segment increases, the PEG segments crystallize as evidenced by multiple crystal reflections that are identical to pure PEG. In addition, the crystallization produces periodic low-angle peaks, which correspond to crystal thickness controlled by the well-defined PEG spacer length. As the temperature increases, a new X-ray scattering peak at q = 2-3 nm$^{-1}$ gradually appears in Na and Cs-neutralized ionomers, while Li-neutralized ionomers exhibit a similar peak across the entire temperature range studied (25 $^{\circ}$C - 150 $^{\circ}$C). The peak intensity in all ionomers increases with temperature and the angular position shifts very slightly to the lower angle. The appearance of an ionomer peak at high temperature is attributed to the increasing extent of microphase separation of ionic groups due to the decreased ability of PEG to solvate ions.